This invention relates to optically amplified lightwave systems and, more particularly, to a method and apparatus for power and wavelength management for a mixed-rate optical data system.
In optically amplified lightwave systems, system performance of a given wavelength channel is determined, to first order, by its optical signal to-noise ratio (SNR). For higher bit rates larger SNRs (consequently higher signal powers) are required. (This can also be explained by realizing that the number of photons/bit required by a receiver is, again to first order, roughly independent of the bit rate; consequently higher bit rates require higher signal powers.) Presently in wavelength-multiplexed (WDM) systems, the bit rates of all channels are the same. Therefore the launch powers of all signals are nominally the same.
Unfortunately under certain circumstances, the transmission of all signal channels at the same data bit rate and at the same launch power level can adversely affect system performance.
In accordance with the present invention, we exploit the reduced SNR requirements for low-bit rate channels to devise a new wavelength channel allocation scheme of a WDM system which increases the number of channels an optical transmission system can support. We assign the wavelengths of low-bit rate channels outside a flat-gain window (i.e., flat-passband region) of the amplifiers. Moreover, our channel allocation scheme has the high-bit rate channels located in the flat-passband region of wavelengths and the lower-bit rate channels located outside this passband region with progressively lower-bit rate channels located farther outside this passband region.
More particularly, in accordance with our invention, a wavelength division multiplexed (WDM) system comprises (1) at least one optical amplifier for amplifying optical wavelengths in at least two regions, each region exhibiting a different transmission gain characteristic for all wavelengths within that region; (2) at least one first-type transmitter, each first-type transmitter transmitting an optical wavelength selected from a first region of said at least two regions and modulated at a data bit rate at or below a first data bit rate and (3) at least one second-type transmitter, each second-type transmitter transmitting an optical wavelength selected from a second region of said at least two regions and modulated at a data bit rate at or below a second data bit rate, where the second bit rate is lower than the first bit rate.
According to one aspect of our invention, the first region is a flat-passband transmission region of the system having a gain characteristic which falls within a predetermined range and the second region has a gain characteristic outside that predetermined range. According to one embodiment, the second region is located in a roll-off region of the system gain characteristic. In yet another embodiment, each second-type transmitter transmits at a wavelength within the second region the data bit rate which is selected so that the output power level for said each wavelength in the second region does not exceed the non-linear threshold level of the transmission fiber of the system.
The method of our invention comprises the steps of (1) determining the transmitted power versus wavelength characteristics of the system for each wavelength used in a WDM system; (2) for at least one wavelength having a transmitted power level within a predetermined range, selecting a transmission data bit rate at or below a first data bit rate; and (3) for at least one wavelength having a transmitted power level outside said predetermined range, selecting a transmission data bit rate at or below a second data bit rate, said second data bit rate being lower than the first data bit rate.